# R (disambiguation)

R, or r, is the eighteenth letter of the English alphabet.
R or r may also refer to:

## Science
### Biology and medicine

* Arginine, an amino acid abbreviated as Arg or R
* ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
* Coefficient of relationship (r), in biology
* Effective reproduction number (R), the number of cases generated by one case in the current state of a population in epidemiology
** Basic reproduction number (R$_0$), the expected number of cases directly generated by one case
* Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
* Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
* Net reproduction rate (R$_0$), the average number of offspring that would be born to a female given conforming conditions
* r, the population growth rate in the r/K selection theory of ecology

### Astronomy

* Spectral resolution ($\mathbb {R}$), in astronomy
* Orangish or K carbon stars (stellar classification: R)

### Physics

* R (cross section ratio), the ratio of hadronic to muonic cross sections
* Electrical resistance (R)
* Roentgen (unit) (R), a unit of measurement for ionizing radiation such as X-ray and gamma rays
* Rydberg constant (R$_∞$, R$_H$), a physical constant relating to energy levels of electrons within atoms
** Rydberg unit of energy (R$_y$), the energy of the photon whose wavenumber is the Rydberg constant

#### Temperature scales

* Rankine scale (°R, °Ra)
* Réaumur scale (°Ré, °Re, °r)
* Rømer scale (°R, °Rø)

### Chemistry

* Gas constant (R), in chemistry
* R, one of two chiral center configurations in the R/S system
* Alkyl group, commonly marked as R in structural formulas
* Side chain (an unspecified group of atoms), in a chemical or structural formula

### Mathematics

* r is the symbol for the radius of a circle
* $\mathbb {R}$ or R, the set of all real numbers
* Pearson product-moment correlation coefficient (r), in statistics
* R (programming language), an environment for statistical computing and graphics
* Ricci curvature
* Γ, the gamma function

## Technology and engineering

* R-value (insulation), a non-SI measure of thermal resistance used in housing insulation
* R-value (soils), property of soils used in pavement design
* Resistor, an electronic component
* R-, a refrigerant numbering system
* R, the ratio between the minimum and maximum stresses in fracture mechanics
* R- or recall-button, now mostly used to switch between two calls

### Computing

* R (programming language), an environment for statistical computing and graphics
* R (complexity), the set of all recursive languages
* IBM System R, an IBM database system
* RequestBot, a service in QuakeNet's IRC services

## Arts and entertainment
### Music

* Ritardando, a term for slowing the tempo down gradually
* R. (R. Kelly album), a 1998 album by R. Kelly
* R° (Ruratia album), a 2002 album by Rurutia
* R (single album), a 2021 single album by Rosé
* Rated R (Queens of the Stone Age album), 2000 album also called R
* R (jp), 2005 J-Pop album by Ryoko Shiraishi

### Film

* R (film), a 2010 film by Danish screenwriter and film director Tobias Lindholm
* R, a character played by John Cleese in The World Is Not Enough
* R, the protagonist of Warm Bodies, based on the book of the same name
* R, a film rating in the Motion Picture Association of America film rating system and in the Canadian Home Video Rating System

### Television

* Rajawali Televisi (RTV), television in Indonesia
* R, the production code for the 1965 Doctor Who serial The Chase

### Video games

* R·Type, an arcade video game
* R: Rock'n Riders, in the list of PlayStation games (M–Z)

## Organisations

* The Crown, Rex (King) or Regina (Queen) in Commonwealth nations
* Ryder (NYSE ticker symbol)
* R (cable operator), a telecommunications company
* R, denoting a member of the US Republican Party

## Transportation

* Nissan Skyline GT-R, commonly referred to as the "R" among Japanese speakers
* R (New York City Subway service)
* Rinkai Line, a railway line operated by the Tokyo Waterfront Area Rapid Transit, labeled R
* Transilien Line R, a line of the Paris transport network
* Reconnaissance aircraft (military designation: R)
* Restricted airspace
* Volkswagen R, the performance division of Volkswagen
* The official West Japan Railway Company service symbol for:
** Hanwa Line.
** San'yō Main Line.

## Other uses

* Registered trademark symbol (®)
* South African rand (R), a currency
* Right (direction)
* Rolled R (IPA: r), the alveolar trill of Spanish, Italian, & Arabic languages
* UTC−05:00 (military time zone code: Romeo)
* Я, a Cyrillic letter similar to R
* Subreddit, user-created areas of interest where discussions on Reddit are organized.